Photo-voltaic solar energy is produced by converting sunlight into electricity through photovoltaic, or solar, cells. These cells are formed of purified silicon, doped during growth with phosphorous and boron to form a p-n junction. Cylinders of the doped silicon are sliced into wafers and plated with a conductive pattern to produce solar cells. The specific configuration of an exemplary silicon solar cell is shown in Milnes, Semi-conductors and Integrated Electronics, Van Nostrand, Reinhold Company, 1980, Section 12.2, incorporated herein by reference.
As light falls on a solar cell, red light penetrates deep and blue light is absorbed just below the surface of the cell. Under illumination by light, the p-n junction tends to develop a junction-depletion region which causes electrons induced in the p-region to move to the n-region, producing electricity. The magnitude of current is described by the Shockley model described in section 12.2.2 of the Milnes text, supra.
In a conventional solar power system, solar energy is absorbed by an array of photo-voltaic solar cells that generate a substantial direct current applied to a battery through a control regulator. In practice, each cell of a battery has an electrical potential of approximately 2.12 volts. The battery cells are connected in series to produce higher working voltages. A 12 volt battery, for example, consists of 6 cells whereas a 120 volt battery constitutes 60 cells.
In a conventional solar cell array, each cell of the battery is connected in series and each photo-voltaic cell is connected in series. Thus, if there are 6 cells in a battery, there are 36 photo-voltaic cells and six battery cells connected in series.
The efficiency of this configuration is relatively low. Bearing in mind that the maximum conversion efficiency of a conventional photo-voltaic cell is on the order of under 10% it is important to maximize energy transfer between photovoltaic cells and battery cells, to make photo-voltaic energy conversion systems practical.